Leafcutter ant

Name and taxonomy

Carl Linnaeus described the species in 1758, in the tenth edition of Systema Naturae. He named it Formica cephalotes and placed it in the genus Formica beside 16 other ants. Johan Christian Fabricius moved it into the new genus Atta in 1804, together with five other species. William Morton Wheeler made A. cephalotes the type species of Atta in 1911. An older genus name, Oecodoma, also used it as a type species, but that name is now a synonym of Atta.

The species belongs to the ant family Formicidae, inside the order Hymenoptera with the bees and wasps. Within the ants it sits in the subfamily Myrmicinae and the tribe Attini, the fungus-growing ants. iNaturalist calls it the hairy-headed leafcutter ant, and Wikidata lists leaf-cutter ant as the plain English name. French speakers call it fourmi céphalote or fourmi grosse. The name cephalotes probably refers to the wide head of the soldier caste.

Evolution and relatives

The tribe Attini holds the ants that grow fungus for food, and A. cephalotes belongs to the subtribe Attina. Like other attine ants, the leafcutter ant is thought to descend from one ancestor that took up fungus farming. Most attine ants are thought to feed their fungus on dead plant matter, and only the leafcutter genera cut fresh leaves. The genus Atta contains the leafcutter ants of the Americas, and A. cephalotes is the species that defines the genus.

Ants, bees and wasps form the order Hymenoptera, and ants sit inside the stinging group Aculeata. Like most Myrmicinae, the leafcutter ant carries a two-segment waist between the thorax and the abdomen. Its closest relatives are probably the other Atta species, such as Atta laevigata, which people eat in Colombia.

Appearance

The worker is red-brown, and a hard exoskeleton covers its body in place of a skeleton. The sources give no measured body length for this species, so this page describes size by caste, not in millimetres. The smallest workers are the minims, which tend the fungus garden and ride on the leaves that larger ants carry. The mediae are workers of middle size, and they do the cutting and carrying. The majors are the largest workers, and their common name, hairy-headed, points to the bristled head of the soldier.

Like other ants, the body has three parts, a head, a thorax and an abdomen, with six legs and two elbowed antennae. Like other ants, only the queen and the males grow wings, and the queen sheds hers after her mating flight. The queen is probably many times the size of a minim, because she must carry eggs for a colony of millions. The fields hold no weight or length for the species, so no figure on this page is a measured one.

Habitat and range

The species lives in the Neotropical region, from Mexico through Central America to Bolivia. Two separate populations live in Amazonas and in north-eastern Brazil. It is a rainforest ant, and one colony usually covers about 200 square metres of the forest floor. Nests reach 7 metres deep. The ants place the nest so that a breeze can carry away the carbon dioxide that the fungus gives off. Without that airflow the gas would build to dangerous levels inside the chambers.

The nest mound can spread over 30 metres, and side mounds reach out to 80 metres from the centre. Inside, the nest holds hundreds of chambers, each one a garden, a nursery or a rubbish store. GBIF lists records from 23 countries, but that count includes zoo colonies in Europe and is not the native range. Costa Rica and Colombia give the most records, which probably reflects where people look rather than where colonies are densest. In Colombia the species is recorded across most departments, from Amazonas in the south to Vichada in the east.

Diet and feeding

The leafcutter ant counts as a herbivore in one sense only: it cuts leaves, but it never eats them. Workers chew the leaf pieces into a mulch and add the mulch to the fungus garden inside the nest. The fungus is Leucoagaricus gongylophorus, and it grows swollen tips called gongylidia. The ants eat these swollen tips, and the fungus is the only food the colony has. About 90 percent of the leaf cutting happens inside the nest, not on the tree.

The ants weed the garden and remove patches that a mould has infected. They carry a bacterium on their bodies, and this bacterium makes an antibiotic that protects the garden. The species is one of the main plant eaters of the Neotropics, and it strips crops as readily as wild trees. Like other leafcutters, a colony probably takes leaves from many plant species and moves on when one tree is stripped. The larvae are thought to feed only on the fungus, as the adults do.

Behaviour and social life

One queen founds and heads each colony, and the colony can grow to 5 million members. Our fact file puts the largest colonies at up to 8 million ants. The work is split between castes by body size, a system that biologists call task partitioning. Minims dig, tend the fungus and care for the brood inside the nest. Minors clean the nest and nurse the young. Mediae are the harvesters: they cut leaves and carry them home along the trails. Majors guard the trails against other insects and do little else.

The oldest workers run the rubbish dump, where the colony discards spent mulch and dead fungus. The rest of the colony keeps these dump workers apart. A dump worker that wanders out is forced back or killed by its nest mates. Dump workers pick up disease and toxins, and they live only half as long as workers of the same age. Minims also ride on the leaves that mediae carry, and from there they fight off parasitic flies. These phorid flies try to lay eggs inside the heads of the leaf carriers.

The colony is active by day and by night. Like other ants, a worker probably spends its whole life inside the colony and never breeds. A colony of millions is thought to work without any central control, because each ant follows simple local rules.

Communication and senses

A forager lays a pheromone trail on the way back from a good tree, and other workers follow the trail out. Like other ants, the leafcutter ant reads these chemical signals with its antennae, which carry most of its sense of smell and taste. Workers probably also touch antennae to pass on the scent of a good leaf, as other ants do. The compound eyes are small, and vision is thought to matter less than smell inside a dark nest.

Like other ants, workers probably sense vibration through the ground and use it as a warning. The colony also uses scent to sort nest mates from strangers, and a dump worker probably carries a scent that marks it as unclean. The ants are thought to sense the carbon dioxide from the fungus, because they build the nest to vent it. A minim on a leaf probably feels a phorid fly land before it sees it, and it snaps at the fly with its jaws.

Reproduction and life cycle

The queen lives up to 20 years, and she is the only ant in the colony that lays eggs. Winged queens and males leave the nest on a mating flight. A new queen carries a small lump of fungus, taken from her home nest, in a pocket in her mouth. She uses that lump to start a new garden, and the garden feeds her first workers. Like other ants, she sheds her wings after the flight and digs her first chamber alone.

Development from egg to adult takes about 4 weeks. Like other Hymenoptera, the young pass through a larva and a pupa before the adult stage, so an adult ant never moults. The queen lays millions of eggs over her life. Workers live for months, not years, and dump workers live half as long as the rest. Which caste a larva becomes is thought to depend on how much food it receives. A colony probably dies when its single queen dies, because no worker can replace her.

Predators and threats

Armadillos, anteaters and birds eat leafcutter ants. Phorid flies attack the workers on the trails and lay their eggs inside the ants' heads. The minims that ride on carried leaves defend the carriers from these flies. Majors guard the trails from other insects. Moulds in the fungus garden are probably the greatest danger to a colony, which is why the ants weed and carry antibiotic bacteria.

The rubbish dump holds disease and toxins, so the colony walls it off together with the workers who run it. No source lists a threat to the species as a whole. Its main enemy is the farmer who treats it as a pest and destroys the nest.

Population and conservation

The IUCN has not evaluated the species, so its code is NE, Not Evaluated. iNaturalist lists no regional conservation status for it either. With colonies of millions and a range from Mexico to Bolivia, it is not a species in trouble. iNaturalist holds 9,696 observations, and GBIF holds records from 23 countries. Those counts suggest a common animal that people notice easily, because its trails cross forest paths.

Farmers regard the species as a crop pest, and they kill colonies rather than protect them. Clearing of rainforest removes colonies, but the ant is thought to persist well on farmland and in gardens. Nobody counts leafcutter colonies, so the true trend of the population is probably unknown.

Relationship with humans

People in the Neotropics meet the leafcutter ant mostly as a pest. A colony can strip crop plants and garden trees, and it takes leaves from many kinds of plant. In Colombia people eat the winged queens of a related species, Atta laevigata, as hormigas culonas. Zoos in Europe keep colonies, which is why GBIF lists records from the Netherlands, Germany and Switzerland.

Scientists study the antibiotic bacteria on the ants, which may point the way to new medicines. A soldier can give a painful bite, but the species is thought to pose no real danger to people. The species is wild, and nobody has domesticated it. A leafcutter column is probably the ant that visitors to a Neotropical forest remember most.

In culture and myth

In Colombia, people fry the winged queens of a related species, Atta laevigata, and eat them as hormigas culonas. That custom is thought to date from before the Spanish conquest, and the season for it falls with the mating flights. In Aztec tradition, the god Quetzalcoatl is thought to have followed an ant to the hidden store of maize. Ants in general appear in Aesop's fable of the ant and the grasshopper, where the ant stands for hard work.

The leafcutter ant appears in many nature films, because a moving line of green leaf pieces is easy to film. Zoo displays of a leafcutter colony seem to draw crowds, and several European zoos keep one. The ant's farming is often used to suggest that agriculture is older than humans.